Drum mixers for asphaltic materials have been developed which include a rotating drum having internal flights for lifting and tumbling aggregate and a bituminous binder to mix the two. A source of high volume heated gas is aligned with the drum for providing the interior of the same with a continuous flow of a hot gaseous medium. In most prior art drum mixers of this type, the heated gases are produced by a burner-blower arrangement at the inlet end of the drum which produces a flame projecting into the drum along its central axis.
As is explained in U.S. Pat. No. 4,039,171, the operation of such drum mixers can result in considerable atmospheric pollution due to the concentrated high temperature gases in and around the open flame. That is, when an open flame projects into the inlet end of the drum mixer, asphaltic materials cannot be introduced near the flame zone of the mixer without causing these materials to vaporize, pyrolize and produce dense smoke. The production of such smoke is associated with the use of those asphalts having relatively low smoking points and is caused by the tumbling of the asphalt through, and consequently subjecting the asphalt to, the extremely high temperatures typically found along the central axis of an operating drum mixer utilizing an open flame at its entrance end. This high temperature zone at the central axis is normally present only near the inlet end of the mixer and eventually dissipates at increasing distances into the mixer from the inlet end. Accordingly, when using low smoking point asphalts, the introduction of the asphalt must oftentimes be made at substantial distances downstream from the inlet end of the mixer. When the asphalt is added at such downstream positions, however, particulate emission is increased for the reason that the aggregate, which contains a certain amount of fines, is tumbled within the drum for a considerable period before it is contacted by the asphalt binder.
The conservation of paving material has been of great interest to the paving industry in recent years. With increased costs of aggregates and asphalts, coupled with lessening supplies of both materials, a need has arisen for new methods and equipment designed to use less of such materials in virgin form. One solution to the material cost and supply problem is the use of old materials removed from existing roadways as an ingredient in a paving mixture being prepared for repaving of existing roadway surfaces. The old, removed materials are crushed and are then introduced into the mixer, usually along with a certain amount of virgin aggregate and asphalt. As the recycled material moves through the drum, the old asphalt binder melts to produce a new mix which can then be used to repave the roadway from which it was taken.
When recycled paving materials are introduced into a drum mixer having an open flame projecting into its inlet end, the production of excessive amounts of pollutants ordinarily occurs due to the fact that the recycled material contains fine particles of asphalt or asphalt-coated aggregate which are subjected to the intense heat of the flame at the entrance end of the drum. As will be understood, this problem cannot be alleviated, as in the case of virgin aggregate, by simply introducing fresh asphalt downstream of the flame since asphalt is inherently present in the recycled material.
In the aforesaid U.S. Pat. No. 4,039,171, a dispersing means is provided within the mixing drum downstream from the burner and extends across the interior cross section of the drum to disperse the heating gases uniformly across the interior of the drum and prevent burning of asphaltic materials and resultant pollution. Preferably, the dispersing means is in the form of a screen extending across the drum. While a dispersing means of this type materially improves the elimination of pollutants and permits the introduction of recycled aggregates and bituminous materials nearer to the burner, it is not altogether satisfactory. In particular, difficulty has been encountered in selecting a material for the screen which is capable of withstanding the extremely high temperatures involved, on the order of 2000.degree. F. and above.